Wireless internet connection problem.

For the last fortnight I've been struggling with my new laptop that I ordered before I finally had the idea to look for a place just like this. I bought a Dell Inspiron 1545 Laptop, and I can't get it to connect to our wireless internet. It came with Vista installed, but I've just upgraded to 7, I couldn't connect with either system though. I'm trying to connect to a SpeadStream 6520 Wireless ADSL Gateway. I've tried adding it using a USB key - and it says that my laptop has been successfully added to the network, but I then can't connect. I've also tried entering in the details manually, still with no success. I was wondering if anyone could help me? I'm so confused with it all now.